The physical constitutions of vehicle occupants who wear seat belts are different to a considerable extent from one occupant to another. Accordingly, it is preferred from the viewpoint of safety that the length of each webbing and the mounting positions of its anchors (especially, the upper fitting for a shoulder webbing) are adjustable in accordance with the physical constitution of the occupant who wears the webbing. The webbing extends over the face or neck area of the occupant if the position of the upper fitting for the shoulder webbing is too high. On the other hand, the webbing passes over his arm instead of extending over his shoulder if the position of the upper fitting for the shoulder webbing is too low. Each of the above cases involves a potential danger that the webbing may not be able to protect the occupant effectively in the event of an emergency.
With the foregoing in view, a variety of adjustable upper-fitting mounting devides has heretofore been proposed. Many of such prior art adjustable upper-fitting mounting devices are, as disclosed for example in U.S. Pat. No. 4,225,185 issued Sept. 30, 1980 to Peter Krzok and U.K. Patent Specification No. 1,486,974 published Sept. 28, 1977 and naming Chrysler United Kingdom Limited as the applicant, equipped with latch members integrally with upper fittings so that the latch members may be brought into most suitable interlocking holes by pushing or pulling the upper fittings. On the other hand, as disclosed in U.K. Patent Application No. 2,081,568A laid open on Feb. 24, 1982 and naming Beyerische Motoren Werke AG as the applicant and U.K. Patent Application No. 2,078,092A laid open on Jan. 6, 1980 and naming P A Rentrop Hubbert & Wagner, Fahrzeugausstauttungen GmbH & Co. KG., a latch member is displaceably provided on a slide member on which an upper fitting is secured. The position of the upper fitting may be adjusted by bringing the latch member into engagement with a desired attachment hole out of a plurality of attachment holes formed in a guide member which guides the slide member. In order to maintain the upper fitting at the mounted position, the latch member is pressed against the engaged position by spring means. In mounting devices of the above sort, there is a potential danger that, if an impact force is applied in the direction extending along the direction of displacement of the latch member in such an event as a vehicle collision for instance, the latch member may be moved against the urging force of the spring means by virtue of an inertia force produced on the latch member per se and may thus be disengaged from the attachment hole. If the above situation should happen, the mounted position of the upper fitting is moved, thereby possibly failing to protect the occupant effectively as mentioned above.